NAND memory array with mismatched cell and bitline pitch

ABSTRACT

Embodiments of the present disclosure describe methods, apparatus, and system configurations for NAND memory arrays with mismatched cell and bitline pitch. Other embodiments may be described and claimed. The bitline pitch is the distance between bitlines. The cell pitch is the distance between cells. The mismatch is bitline spacing that is different from cell spacing.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/931,784, filed Nov. 3, 2015, which is a continuation of U.S.patent application Ser. No. 13/993,312, filed Jun. 11, 2013, which is anational phase entry under 35 U.S.C. § 371 of International ApplicationNo. PCT/US2011/052846, filed Sep. 22, 2011, entitled “NAND MEMORY ARRAYWITH MISMATCHED CELL AND BITLINE PITCH,” and the entire contents anddisclosures of which are hereby incorporated by reference in theirentireties.

FIELD

Embodiments of the present disclosure generally relate to the field ofintegrated circuits, and more particularly, to a NAND memory array withmismatched cell and bitline pitch.

BACKGROUND

In a NAND memory array, all the memory cells in a page are programmedand read at the same time. So, in principle, the larger the page size,the greater the parallelization of the program/read operations. Thiswill result in higher data throughput provided that the program/readoperations themselves are not greatly degraded as a result of the largerpage size. Historically, NAND page size has steadily increased fromgeneration to generation in order to support higher and higher datathroughput, even though the program/read timing has been degrading ingeneral. The increases in page size are possible due to the fact thatthe cell pitch scales down from generation to generation.

A page will include all of the memory cells (in an all-bitline (ABL)architecture) or half of the memory cells (in a shielded bitline (SBL)architecture) along a single wordline in an array plane. Limitations ofa size of a die limit the absolute length of the wordlines (i.e., thewidth of an array plane). That sets an upper limit on the number ofmemory cells that can be placed along a single wordline. So, withmatched bitline and cell pitch, the page size that can be supported islimited for a given cell pitch.

The limitation on page size has not been a significant issue for NANDbecause the cell pitch has been steadily decreasing from generation togeneration as a result of cell scaling, which enables page sizeincreases within the die-size limitation. However, as conventional NANDscaling comes to an end, future NAND scaling may be achieved bythree-dimensional (3-D) NAND memory arrays.

In 3-D NAND memory arrays, the cell size in the wordline direction islimited by a cell channel thickness (in the form of a pillar or line), agate stack thickness (tunnel oxide, charge trapping layer, and blockingoxide), and a gate electrode thickness. As a result, the cell pitch inthe wordline direction will be significantly bigger than conventional,i.e., 2-D NAND memory arrays. While density-wise, the larger cell pitchcan be compensated by the fact that multiple layers of cells are stackedon top of one another, the page size will have to come down because ofthe larger cell pitch, with everything else being equal (diearchitecture, package size, etc.). Thus, the data throughput of 3-D NANDmemory arrays may be significantly degraded compared to 2-D NAND memoryarrays, limiting their competitiveness.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be readily understood by the following detaileddescription in conjunction with the accompanying drawings. To facilitatethis description, like reference numerals designate like structuralelements. Embodiments are illustrated by way of example and not by wayof limitation in the figures of the accompanying drawings.

FIG. 1 illustrates a 2-D memory array in accordance with someembodiments.

FIGS. 2 and 3 illustrate a 3-D memory array in accordance with someembodiments.

FIG. 4 illustrates a flowchart of a method of performing an accessoperation in accordance with some embodiments.

FIG. 5 illustrates an example system in accordance with someembodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof wherein like numeralsdesignate like parts throughout, and in which is shown by way ofillustration embodiments that may be practiced. It is to be understoodthat other embodiments may be utilized and structural or logical changesmay be made without departing from the scope of the present disclosure.Therefore, the following detailed description is not to be taken in alimiting sense, and the scope of embodiments is defined by the appendedclaims and their equivalents.

Various operations may be described as multiple discrete actions oroperations in turn, in a manner that is most helpful in understandingthe claimed subject matter. However, the order of description should notbe construed as to imply that these operations are necessarily orderdependent. In particular, these operations may not be performed in theorder of presentation. Operations described may be performed in adifferent order than the described embodiment. Various additionaloperations may be performed and/or described operations may be omittedin additional embodiments.

For the purposes of the present disclosure, the phrase “A and/or B”means (A), (B), or (A and B). For the purposes of the presentdisclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B),(A and C), (B and C), or (A, B and C).

The description may use the phrases “in an embodiment,” or “inembodiments,” which may each refer to one or more of the same ordifferent embodiments. Furthermore, the terms “comprising,” “including,”“having,” and the like, as used with respect to embodiments of thepresent disclosure, are synonymous.

FIG. 1 schematically illustrates a memory array 100 in accordance withsome embodiments. The memory array 100 may be a 2-D NAND memory arrayhaving wordlines 104 and active area sections, e.g., active area strips108, arranged orthogonal to one another, with the wordlines 104horizontally traversing the memory array 100, and the active area strips108 vertically traversing the memory array 100. Perspective-baseddescriptors, such as vertical and horizontal, may be used to facilitatediscussion. These descriptors do not limit the implementations ofembodiments of the disclosure.

Memory cells 112 are disposed at an overlap of the active area strips108 and wordlines 104. The memory cells 112 may be separated from eachother by a characteristic cell pitch (CP), which, as used herein, may bea distance between adjacent memory cells in a direction of the wordlines104. The memory array 100 may further include bitlines 116 arrangedorthogonal to the wordlines 104. The bitlines 116 may be separated fromeach other by a characteristic bitline pitch (BP), which, as usedherein, may be a distance between adjacent bitlines in a direction ofthe wordlines 104. As will be described below, various embodiments ofthe disclosure decouple the bitline pitch from the cell pitch (e.g.,make the bitline pitch less than the cell pitch) in order to allow moreof the memory cells 112 to be selected for each access (e.g., program orread) operation.

The memory array 100 may further include a number of electrical linesarranged parallel to the wordlines 104 that may be selectivelycontrolled to access various sets of the memory cells 112. Theelectrical lines may include select gate drains (SGDs), e.g., SGD_n+1120, SGD_n 124 a and 124 b, and SGD_n−1 128; select gate source (SGS)132 a and 132 b; and common source line (CSL) 136. Electrical contacts140 may serve to electrically couple bitlines 116 to the active areastrips 108.

The memory array 100 may include a first sub-block 144 a that has NANDstrings 148 a corresponding with a first subset of bitlines. The memoryarray 100 may also include a second sub-block 144 b that has NANDstrings 148 b that correspond with a second subset of bitlines.

In various embodiments, each block of the memory array 100 may include anumber of sub-blocks that is based on a ratio of the cell-to-bitlinepitch. For example, block 144 includes two sub-blocks due to a 2:1cell-to-bitline pitch ratio. For a general cell-to-bitline pitch ratioof n:1, there may be n sub-blocks per block.

The first and second subsets of bitlines may be non-overlapping subsets.In some embodiments, the first subset of bitlines may be interleavedwith the second subset. For example, the first subset may include theeven bitlines (e.g., second bitline, fourth bitline, sixth bitline,etc.), while the second subset includes the odd bitlines (e.g., firstbitline, third bitline, the bitline, etc.).

The numbering of the bitlines may start from the left side of the pagein FIG. 1.

A NAND string may include memory cells between an SGD and an SGS along agiven active area strip. As can be seen, the NAND strings 148 a and theNAND strings 148 b may be offset with respect to one another. In thisembodiment, every bitline may be electrically coupled to every othersub-block. So, for example, the first bitline may not be electricallycoupled with sub-block 144 a, may be electrically coupled with sub-block144 b, may not be electrically coupled with a sub-block below SGD_n−1128, and so forth. This may allow for the simultaneous driving ofsub-block 144 a and sub-block 144 b to perform various accessoperations.

As used herein, simultaneous operations may be operations that areentirely concurrent with one another, e.g., start and end at the sametimes, or partially concurrent with one another, e.g., have differentstart and/or end times.

Reducing the bitline pitch to half of the cell pitch and offsettingadjacent NAND strings, as shown, may effectively double the page sizewith respect to conventional memory arrays in which the bitline pitchand the cell pitch are the same. A page, in this context, may refer toall the memory cells (in an ABL architecture) along a pair of wordlines,with a first wordline from a first sub-block, e.g., sub-block 144 a, anda second wordline from a second sub-block, e.g., sub-block 144 b. Thepair of wordlines may be electrically coupled with one another at anedge of the array and be driven by a single driver. In embodimentshaving n sub-blocks, a page may include all the memory cells (in an ABLarchitecture) along n wordlines, one per sub-block, with the n wordlinesbeing electrically coupled with one another at an edge of the array anddriven with the single driver.

Operating sub-block 144 a simultaneously with sub-block 144 beffectively provides the block 144 with a size that is twice that of aconventional memory block. The number of pages within block 144 may bethe same as a conventional memory block (because the wordlines of thetwo sub-blocks can be driven together), but the page size doubles as aresult of the 2× denser bitlines 116. With twice the number of bitlines116 and twice the page size, it may be desirable for the sensingcircuitry coupled with the memory array 100 to also be doubled.

Advantages of a mismatched cell and bitline pitch may be less dramaticin a 2-D NAND memory array, in which a cell pitch may be at or close toprocess capability for a given generation, than in a 3-D NAND memoryarray, in which cell pitch is limited by the cell characteristics andcan be much greater than the process capability.

In a 3-D NAND memory array such as a pipe-shaped bit cost scalable(P-BiCS) memory array, cell pitch in the wordline direction can beexpressed as: cell pitch=pillar diameter+2×gate dielectric stackthickness+control gate (e.g., wordline) thickness between cells.

With optimistic assumptions of 20 nanometers (nm) for a pillar diameter,25 nm for a gate dielectric stack thickness, and 20 nm for a controlgate thickness between cells, a cell pitch would be approximately 90 nm,which is much greater than current process capability at approximately20 nm-29 nm half-pitch. It is possible that cell pitch in a 3-D NANDmemory array may be even bigger than the above estimate when the impactof the edge profile on cell uniformity, critical dimension, andregistration (e.g., alignment tolerance) margins are considered.Therefore, patterning bitlines at a pitch that is less than a cell pitchmay be quite practical for a 3-D NAND memory array.

FIGS. 2 and 3 illustrate a memory array 200 from a perspective andtop-plan view, respectively, in accordance with some embodiments. Thememory array 200 may be a P-BiCS array; however, other embodiments mayinclude other types of 3-D NAND memory arrays.

The memory array 200 includes wordlines 204 and active area sections,e.g., active area pillars 208, arranged orthogonal to one another, withthe wordlines 204 traversing the memory array 100 in a y-direction andthe active area pillars 208 traversing the memory array 200 in az-direction. The memory array 200 may include memory cells 212 disposedat intersections of the active area pillars 208 and the wordlines 204.

The memory array 200 may also include bitlines 216 disposed at the topof the memory array 200 and traversing the memory array 200 in anx-direction. Thus, the bitlines 216 may be arranged orthogonal to boththe wordlines 204 and the active area pillars 208.

In this embodiment, similar to embodiment described above with respectto FIG. 1, the bitline pitch may be decoupled from, e.g., less than, thecell pitch.

The memory array 200 may also include electrical lines arranged parallelto the wordlines 204 that may be selectively controlled to accessvarious sets of the memory cells 212. The electrical lines may includeSGDs, e.g., SGD_n−2 220, SGD_n−1 224, SGD_n 228, and SGD_n+1 232; SGS234 a, 234 b, 234 c, 234 d, and 234 e; and CSL 236. Electrical contacts240 may serve to electrically couple bitlines 216 to the active areapillars 208.

The memory array 200 may include a first sub-block 244 a that has NANDstrings 248 a corresponding with a first subset of the bitlines 216,e.g., the even bitlines (e.g., second bitline, fourth bitline, sixthbitline, etc.). The number of the bitlines may be with respect to theview shown in FIG. 3, with the numbering starting from the left side ofthe page. The memory array 200 may also include a second sub-block 244 bthat has NAND strings 248 b corresponding with a second subset ofbitlines, e.g., the odd bitlines (e.g., first bitline, third bitline,fifth bitline, etc.). The first and second subsets may benon-overlapping subsets and may be interleaved with respect to oneanother.

A NAND string, in this embodiment, may include memory cells between anSGD and an SGS on a given pair of active area pillars. For example, theNAND strings 248 a may include memory cells of active area pillars thatare disposed between the SGS 234 c and an underlying substrate 252 andactive area pillars between the substrate 252 and the SGD_n 228. Theseactive area pillars may be electrically coupled with one another by anelectrical connector 256 within the substrate 252. The electricalconnector 256 may sometimes be referred to as a pipe connection (PC).Thus, one NAND string may include eight memory cells as shown in FIG. 2.

The NAND strings 248 a and 248 b may be offset with respect to oneanother. Thus, similar to the memory array 100, memory array 200 mayprovide for the simultaneous driving of the sub-blocks 244 a and 244 bto perform various access operations.

While the above embodiments describe a bitline pitch that is half of acell pitch, other embodiments may have other mismatched bitline and cellpitches. Various embodiments may include any ratio of thecell-to-bitline pitch. For example, the bitline pitch may be 1/n of thecell pitch, where n is an integer greater than one.

While the above embodiments describe a cell-to-bitline pitch as binary,i.e., 2:1, other embodiments may have non-binary cell-to-bitlinepitches. In a binary digital system, it may be preferable to have abinary page size, e.g., total number of bitlines in an ABL architecture,to facilitate the addressing, I/O timing, etc. If the cell-to-bitlinepitch ratio is also binary, this may result in a binary number of cellsfully coupled with the bitlines. However, an embodiment having anon-binary cell-to-bitline pitch may be used, with a slightover-provision of memory cells, as maintaining a binary number ofbitlines may be sufficient to ensure a binary page size.

For example, consider a desired page size of 16-bits. Working off abinary 2:1 cell-to-bitline pitch ratio, an 8-cell wide array may be usedwith the bitlines coupled with every other sub-block. To implement a 3:1cell-to-bitline pitch ratio, a 5-cell wide array (5>16/3) may be usedwith the bitlines coupling with every one in three sub-blocks. In thiscase, there may be a few NAND strings at an edge of the array that willnot be coupled with any bitlines. However, considering a practical pagesize of 8 kilobytes (kB) or 16 kB, the wasted cells are a smalloverhead.

With mismatched bitline and cell pitch, the page size that can besupported by the technology may only be limited by the bitline pitch,not the cell pitch. As no high-aspect ratio etch is needed for bitlineconstruction in 3-D NAND memory arrays, the bitline pitch within sucharrays may be able to match bitline pitch of 2-D NAND memory arrays.Thus, the page size of the 3-D NAND memory array may be similar to 2-DNAND memory array, without having to be smaller to accommodate thelarger cell pitch associated with the 3-D memory array. Even if the cellpitch of the 3-D memory arrays does not further reduce, a page size maybe further scaled if the bitline pitch can be scaled through moreadvanced lithography.

FIG. 4 illustrates a method 400 of performing an access operation with amemory array in accordance with some embodiments. The method 400 may beused with memory array 100 or memory array 200. The method 400 mayinclude, at block 404, simultaneously driving first and secondsub-blocks. The first and second sub-blocks may be physically and/orelectrically adjacent to another and may represent one block of memorycells. Two sub-blocks may be simultaneously driven by energizingappropriate select gates. For example, with respect to memory array 100,sub-block 144 a and sub-block 144 b may be simultaneously driven byenergizing SGD_n 124 a, SGD_n 124 b, and SGS 132 a and 132 b. Withrespect to memory array 200, sub-block 244 a and sub-block 244 b may besimultaneously driven by energizing SGD_n 228 and SGS 234 c and 234 d.

The method 400 may continue, at block 408, with accessing memory cellson a first subset of bitlines, e.g., even bitlines, of the firstsub-block. With respect to memory array 100, the memory cells of thesub-block 144 a may be accessed by energizing the first subset ofbitlines and one or more wordlines of the sub-block 144 a. With respectto memory array 200, the memory cells of sub-block 244 b may be accessedby energizing the first subset of bitlines and selected wordlines of thesub-block 244 b.

The method 400 may also include, at block 412, accessing memory cells ona second subset of bitlines, e.g., odd bitlines, of the secondsub-block. With respect to memory array 100, the memory cells of thesub-block 144 b may be accessed by energizing the second subset ofbitlines and one or more wordlines of the sub-block 144 b. With respectto memory array 200, the memory cells of sub-block 244 a may be accessedby energizing the second subset of bitlines and selected wordlines ofthe sub-block 244 a. In various embodiments, blocks 408 and 412 may beperformed simultaneously with one another and with block 404.

While the above embodiment describes a method with respect to twosubsets of bitlines, other embodiments may include greater numbers ofsubsets, e.g., when the cell-to-bitline pitch ratio is greater than 2:1.Furthermore, while the above embodiment describes a method of drivingtwo sub-blocks simultaneously with one another, other embodiments mayinclude driving more than two sub-blocks simultaneously with oneanother. For example, in an embodiment with a block having n sub-blocks,up to n sub-blocks may be driven simultaneously with one another.

The memory arrays and methods described herein may be implemented into asystem using any suitable hardware and/or software to configure asdesired. FIG. 5 illustrates, for one embodiment, an example system 500comprising system control logic 508 coupled to one or more processor(s)504; a memory device 512; one or more communications interface(s) 516;and input/output (I/O) devices 520.

The memory device 512 may be a non-volatile computer storage chip thatincludes the memory array 100 or the memory array 200. In addition tothe memory array, the memory device 512 may include a package, havingthe memory array disposed therein, driver circuitry (e.g., drivers),input/output connections to electrically couple the memory device 512with other components of the system 500, etc. The memory device 512 maybe configured to be removably or permanently coupled with the system500.

Communications interface(s) 516 may provide an interface for system 500to communicate over one or more network(s) and/or with any othersuitable device. Communications interface(s) 516 may include anysuitable hardware and/or firmware. Communications interface(s) 516 forone embodiment may include, for example, a network adapter, a wirelessnetwork adapter, a telephone modem, and/or a wireless modem. Forwireless communications, communications interface(s) 516 for oneembodiment may use one or more antennas to communicatively couple thesystem 500 with a wireless network.

For one embodiment, at least one of the processor(s) 504 may be packagedtogether with logic for one or more controller(s) of system controllogic 508. For one embodiment, at least one of the processor(s) 504 maybe packaged together with logic for one or more controllers of systemcontrol logic 508 to form a System in Package (SiP). For one embodiment,at least one of the processor(s) 504 may be integrated on the same diewith logic for one or more controller(s) of system control logic 508.For one embodiment, at least one of the processor(s) 504 may beintegrated on the same die with logic for one or more controller(s) ofsystem control logic 508 to form a System on Chip (SoC).

System control logic 508 for one embodiment may include any suitableinterface controllers to provide for any suitable interface to at leastone of the processor(s) 504 and/or to any suitable device or componentin communication with system control logic 508. The system control logic508 may move data into and/or out of the various components of thesystem 500.

System control logic 508 for one embodiment may include a memorycontroller 524 to provide an interface to the memory device 512 tocontrol various access operations such as those described above withrespect to the method 400 of FIG. 4. The memory controller 524 mayinclude control logic 528 that is specifically configured to control thememory device 512 as described herein. In various embodiments, thecontrol logic 528 may include instructions stored in a non-transitorycomputer readable medium (e.g., the memory device 512 or othermemory/storage) that, when executed by at least one of the processor(s)504 cause the memory controller 524 to perform the above-describedoperations.

In various embodiments, the I/O devices 520 may include user interfacesdesigned to enable user interaction with the system 500, peripheralcomponent interfaces designed to enable peripheral component interactionwith the system 500, and/or sensors designed to determine environmentalconditions and/or location information related to the system 500. Invarious embodiments, the user interfaces could include, but are notlimited to, a display, e.g., a liquid crystal display, a touch screendisplay, etc., a speaker, a microphone, one or more digital cameras tocapture pictures and/or video, a flashlight (e.g., a light emittingdiode flash), and a keyboard. In various embodiments, the peripheralcomponent interfaces may include, but are not limited to, a non-volatilememory port, an audio jack, and a power supply interface. In variousembodiments, the sensors may include, but are not limited to, a gyrosensor, an accelerometer, a proximity sensor, an ambient light sensor,and a positioning unit. The positioning unit mayadditionally/alternatively be part of, or interact with, thecommunication interface(s) 516 to communicate with components of apositioning network, e.g., a global positioning system (GPS) satellite.

In various embodiments, the system 500 may be a mobile computing devicesuch as, but not limited to, a laptop computing device, a tabletcomputing device, a netbook, a smartphone, etc.; a desktop computingdevice; a workstation; a server; etc. The system 500 may have more orless components, and/or different architectures.

Although certain embodiments have been illustrated and described hereinfor purposes of description, a wide variety of alternate and/orequivalent embodiments or implementations calculated to achieve the samepurposes may be substituted for the embodiments shown and describedwithout departing from the scope of the present disclosure. Thisapplication is intended to cover any adaptations or variations of theembodiments discussed herein. Therefore, it is manifestly intended thatembodiments described herein be limited only by the claims and theequivalents thereof.

What is claimed is:
 1. A method to manufacture a NAND memory arraycomprising: forming a plurality of wordlines; creating a first stack ofcells, with creating individual cells where a first active area sectionpasses through individual wordlines, the first stack of cells to coupleelectrically a first bitline; and creating a second stack of cellssubstantially parallel to the first stack of cells, with individualcells where a second active area section passes through individualwordlines, the second stack of cells to couple electrically a secondbitline different from the first bitline, the second bitlinesubstantially parallel to the first bitline, wherein the second stack ofcells being a stack of cells physically closest to the first stack ofcells; wherein a distance as measured from the first stack of cells tothe second stack of cells is greater than a distance as measured fromthe first bitline to the second bitline.
 2. The method of claim 1,wherein creating the first stack of cells and creating the second stackof cells comprises forming a three dimensional NAND memory array.
 3. Themethod of claim 2, wherein creating the first stack of cells andcreating the second stack of cells comprise creating vertical NANDstrings.
 4. The method of claim 3, wherein creating the vertical NANDstrings comprises forming multiple memory cells disposed in a verticalstack and coupled electrically between a common select line and a stringselect line.
 5. The method of claim 1, wherein forming the plurality ofwordlines comprises creating multiple conductive lines disposed in aplane parallel to a substrate of the NAND memory array.
 6. The method ofclaim 5, wherein forming the plurality of wordlines comprises creating avertical stack of multiple layers of wordlines, the multiple layersparallel to the substrate and parallel to each other, the multiplelayers of wordlines to intersect the first active area sections to forma vertical stack of memory cells.
 7. The method of claim 1, wherein thedistance as measured from the first stack of cells to the second stackof cells is approximately twice the distance as measured from the firstbitline to the second bitline.
 8. The method of claim 1, wherein thefirst bitline is also to be in electrical contact with a third stack ofcells.
 9. The method of claim 8, wherein the second bitline is to be inelectrical contact with a fourth stack of cells, wherein the first andthird stacks of cells are in one sub-block and the second and fourthstacks of cells are in another sub-block.
 10. The method of claim 9,wherein the first bitline and the second bitline couple electrically toone sub-block or another, and not both sub-blocks.
 11. The method ofclaim 10, wherein the NAND memory array comprises a NAND memory array inaccordance with an all bitline (ABL) architecture.
 12. The method ofclaim 10, wherein the NAND memory array comprises a NAND memory array inaccordance with a shielded bitline (SBL) architecture.
 13. The method ofclaim 1, wherein the distance as measured from the first stack of cellsto the second stack of cells comprises a distance as measured from acenter of where the first active area section passes through theindividual wordlines for the first stack of cells to a center of wherethe second active area section passes through the individual wordlinefor the second stack of cells.
 14. The method of claim 1, wherein thedistance as measured from the first bitline to the second bitlinecomprises a distance from a center of the first bitline to a center ofthe second bitline.